The present invention is generally directed to a novel on-demand printing apparatus capable of printing indicia, such as bar codes, text, graphics and the like, on a print medium, such as labels, tags, tickets and the like.
On-demand multicolor printers are well known in the prior art and are used in many applications to imprint a continuous print medium such as labels, tags and tickets. These applications include bar code printers, ticket printers and garment tag printers. In such printers, the print medium is conveyed through a print station and indicia is printed thereon as the print medium passes a printhead.
Such printing may be performed by a variety of printing techniques, such as impact, ink jet, laser, and thermal transfer printing. At the time of this disclosure, thermal transfer printing is the most widely used printing technology.
In a thermal transfer printing process, a thermally reactive ribbon is disposed between a thermal printhead and the print medium. The thermal printhead has a plurality of heating elements thereon that can be selectively energized. As the thermally reactive ribbon is heated, ink is transferred from the ribbon onto the print medium forming indicia thereon.
To print color, a plurality of thermal transfer print stations are concatenated together, as described in U.S. Pat. No. 5,675,369, wherein each thermal transfer print station contains a stationary printhead having a width at least as wide as the print medium being printed on. Each thermal transfer print station is actuatable for applying a monochromatic image to the print medium. The monochromatic image printed by each print station can be kept either separate or mixed together on the print medium allowing for a large gamut of colors to be printed on the print medium.
Ink jet printing utilizes a printhead having a plurality of ejection nozzles for ejecting ink onto a print medium to form indicia thereon. A prior art bubble jet printer manufactured by Canon.RTM., which is similar to the color printer described in U.S. Pat. No. 5,675,360, replaces each thermal transfer print station with an ink jet print station. Each ink jet print station contains a stationary printhead having the approximate width of the print medium being imaged as well as associated printhead maintenance hardware and electronics.
There are advantages and disadvantages to each of two technologies listed above.
Thermal transfer printing technology generally yields the highest quality image especially when printing machine readable symbologies, such as bar codes. Thermal transfer technology also yields highly durable images, prints very fast, and is robust for harsh industrial printing environments.
Unfortunately, thermal transfer technology is extremely wasteful of ribbons, costly to run, and poor for the environment when printing multiple colors due to ribbon wastage. Ribbon saving means incorporated in these printers helps to decrease the amount of wasted ribbon however, depending on the format of the printed indica, prior art ribbon saving techniques may not be very effective. In addition, incorporating multiple thermal transfer print stations in a printer is very costly and, likewise, renders these types of printers much more expensive then their monochromatic counterparts that only require one thermal transfer print station.
Ink jet printing technology has the key advantage of efficiency. Ink jet printheads consume less power than thermal transfer printheads and only spray ink where required, eliminating generation of wasted ribbons and ink. Print speeds of printers incorporating stationary ink jet printheads, such as the Canon.RTM. printer described before, are approximately the same as thermal transfer printers, although, at least theoretically, the ink jet printers can print at much higher speeds.
The disadvantages of using ink jet technology in on-demand printers is the reliability of the printheads and poor print quality. Most notably, print quality is much lower on printers incorporating stationary ink jet printheads since deviations in ink jet nozzle directionality causes striations in the printed image. Striations may also be caused by clogged or damaged nozzles that will not eject droplets of ink when energized. Inoperative nozzles are especially detrimental when printing machine readable symbologies such as horizontally oriented bar codes since bar and space widths may be inadvertently altered.
The limitations of ink jet technology in on-demand printers described heretofore can be eliminated by using a disposable scanning ink jet printhead and interleaving algorithms which are well known in the art and described in U.S. Pat. No. 5,686,944. Such disposable scanning ink jet printhead, in a preferred embodiment, may have an ink reservoir thereon. Using a disposable ink jet printhead reduces the risk of printhead damage and increases printer robustness because the printheads can be periodically and inexpensively replaced before or immediately after damage to the printhead. A disadvantage to scanning ink jet printheads is the resulting reduction in print speed which limits their use in on-demand printing applications.
The multicolored printers discussed above have not been well accepted by consumers primarily because of excessive equipment costs in both the thermal transfer and ink jet printer types, consumables costs in the case of thermal transfer printers, and low print quality and reliability in the case of stationary ink jet printers.
For the foregoing reasons, an on-demand color printing apparatus is needed that can be manufactured at a low cost; leverages the quality and durability of thermal transfer printing when printing machine readable symbologies and other critical indicia; leverages the high print speed of thermal transfer printing when only monochrome thermal transfer printing is required; leverages the print quality and reliability of ink jet printing using scanning ink jet printheads; and has the efficiency and environmental friendliness of ink jet technology for printing multicolored indicia when desired on a print medium without causing a major reduction in print speed for most image formats. The present invention provides such a novel printing apparatus which presents these features and advantages and which overcomes the problems in the prior art. These will become apparent upon a reading of the attached specification in combination with an examination of the drawings.